This may explain an odd radio call I had from Luton. Routing CFD to BIG we asked for zone transit through Luton but were refused. I routed east making sure we were east of Stevenage and the 350 or 360 radial (I can't remember without looking at a map) to BKP to ensure we were clear.
The controller came onto us 3 times to request we stay clear of controlled airspace. I was sure we were clear but he did not seem to think so.